A Bio‐Based Supramolecular Adhesive: Ultra‐High Adhesion Strengths at both Ambient and Cryogenic Temperatures and Excellent Multi‐Reusability

Abstract Developing high‐performance and reusable adhesives from renewable feedstocks is of significance to sustainable development, yet it still remains a formidable task. Herein, castor oil, melevodopa, and iron ions are used as building blocks to construct a novel bio‐based supramolecular adhesive (BSA) with outstanding adhesion performances. It is prepared through partial coordination between melevodopa functionalized castor oil and Fe3+ ions. Noncovalent interactions between adherends and the catechol unit from melevodopa contribute to reinforcing adhesion, and the metal‐ligand coordination between catechol and Fe3+ ions is utilized to strengthen cohesion. By combining strong adhesion and tough cohesion, the prepared BSA achieves an adhesion strength of 14.6 MPa at ambient temperature, a record‐high value among reported bio‐based adhesives as well as supramolecular adhesives to the best of knowledge. It also outperforms those adhesives at cryogenic temperature, realizing another record‐high adhesion strength of 9.5 MPa at −196 °C. In addition, the BSA displays excellent multi‐reusability with more than 87% of the original adhesion strength remaining even after reuse for ten times. It is highly anticipated that this line of research will provide a new insight into designing bio‐based adhesives with outstanding adhesion performances and excellent multi‐reusability.

Lap shear tests were conducted by using a INSTRON 68TM-5 universal testing machine with a strain rate of 10 mm/min at room temperature or at -196 °C. Adhesive films were cut into rectangle shapes (25 mm × 12.5 mm × 0.2 mm) and placed between two identical substrates (100 mm × 25 mm × 1.6 mm) with an overlap area of 312.5 mm 2 (25 mm × 12.5 mm). In particular, the thickness of the aluminium oxide sheet changed into 4.0 mm due to its brittle nature. Two paper clips held substrates together and two stainless steel wires with a thickness of 0.2 mm were used to control the thickness of the adhesive while heating. The thermal treatment was performed in an oven at 100 °C for 5 min. Prior to the lap shear test, the bonded sheets were cooled down to the room temperature and stood for another 24 hours. Particularly, for lap shear tests at -196 °C, the bonded sheets were immersed in liquid nitrogen prior to tests to lower the temperature to -196 °C and they were also sprayed with liquid nitrogen during tests to keep them at -196 °C.
The stress-strain curves were measured by using a INSTRON 68TM-5 universal testing machine with a strain rate of 50 mm/min at room temperature. Samples were cut into a rectangle shape (60 mm × 10 mm × 0.2 mm) and the gauge length was around 25 mm. The Young's modulus was calculated as the initial slope of the linear stage of the stress-strain curve. Cyclic tensile tests were performed at a strain of 4% without the rest.
The adhesion force was measured on a Cypher VRS atomic force microscopy (AFM) machine utilizing Oxford model AC240TS-R3 tips at room temperature. Igor Pro 6.37 data analysis software was used to process images and analyze force curves. To obtain the adhesion force of each BSAx, the force mapping mode study was performed, which was based on recording the force-displacement curves of 256 spots at a random area of BSAx films. As illustrated in Figure S6, for each spot, the value of the adhesion force is taken as the difference between the horizontal and minimum values on the retraction curves. For each measurement, the scanning area was fixed at 400 μm 2 (20 μm × 20 μm) and force-displacement curves were recorded with a rate of 0.5 or 1.0 Hz. For each BSAx, the value of the adhesion force was taken as the average value of that of 256 spots and the measurement was repeated for at least 3 times.
The thermal mechanical analysis (TMA) was conducted on a TMA 402 F1/F3 Hyperion machine. Rectangular samples (20 mm × 7 mm × 0.2 mm) were heated from -150 to 25 °C with a heating rate of 5 °C/min and a static force of 0.05 N. The mean linear thermal coefficient of expansion was measured as the slope between endpoints of the curve.
The dynamic mechanical analysis (DMA) was conducted on a TA instrument Discovery 850 machine. Rectangular samples (30 mm × 7 mm × 0.2 mm) were heated from 20 to 110 °C or from -150 to 0 °C with a heating rate of 5 °C/min and a constant frequency of 1 Hz.
Rheology tests were performed on a TA Discovery HR-2 rheometer using the parallel plate geometry (25 mm diameter). Samples were heated from 75 to 110 °C with a rate of 5 °C/min and cooled from 110 to 75 °C with the same rate. A constant strain of 0.1% and a constant angular frequency of 0.5 rad/s were used.
After being concentrated, the mixture was distilled at 70 °C to remove pyridine. Then the mixture was diluted by DCM (300 mL) and washed by brine (3 x 200 mL). The collected organic phase was dried by Na2SO4, filtrated, and concentrated. Then the resultant sticky solid was dissolved by chloroform (60 mL), and added with L-3,4-dihydroxyphenylalanine methyl ester hydrochloride (11.8 g, 48 mmol) and TEA (5.2 g, 54 mmol). The mixture was stirred at room temperature for 4 hours and then concentrated. The resultant sticky solid was dissolved by DCM (300 mL) and washed by brine (3 x 200 mL). The organic phase was collected and dried by Na2SO4. After being filtrated and concentrated, the resultant crude product was

Preparation of BSAx.
The BSA0.35 was prepared by the following steps. The melevodopa functionalized castor oil (1 g, 0.6 mmol) was dissolved by MA (6 mL). Then the mixture was added with FeCl3 (0.103 g, 0.64 mmol) and TEA (0.368 g, 3.6 mmol). After standing at room temperature for 12 hours, the resultant gel was dried under vacuum at 50 °C for 6 hours to remove MA and TEA, getting 1.05 g BSA0.35 with a yield of 95%. The BSA0.25, 0.30, 0.40, and 0.45 were prepared through the similar procedures but with a different dosage of FeCl3.

Multi-reuse experiments.
After usual lap shear tests, the separated stainless steel sheets with fractured BSA0.35 films remained were clamped together again and placed in an oven at 100 °C for 5 min. After standing at room temperature for 24 hours, the second adhesion strength of BSA0.35 was recorded through lap shear tests. The above bonding-testing procedures were repeated for another eight times to get the whole ten adhesion strengths of BSA0.35.

Solvent resistance experiments.
Solvent resistance experiments were performed by soaking BSA0.35 bonded stainless steel specimens in different kinds of solvents, including water, artificial sea water, acid solution (pH = 1), alkaline solution (pH = 14), hexane, ethanol, and acetonitrile for 24 hours at room temperature. Lap shear tests were conducted immediately without any treatment.

Statistical Analysis
The data obtained was used without pre-processing unless noted otherwise. Data was expressed as mean ± standard deviation. The sample size (n) was 3 for each data point. Probability (p) values were determined by one-way analysis of variance using Microsoft Excel.
Significance levels were indicated as *p < 0.05, **p < 0.01, and ***p < 0.001.      Figure S13. Cyclic tensile test curves of BSA0.35. Figure S14. The digital image of BSA0.35 bonded glass sheets exposed to an 808 nm nearinfrared light with a power density of 2 W/cm 2 . Figure S15. The temperature-time curve of BSA0.35 bonded glass sheets exposed to an 808 nm near-infrared light with a power density of 2 W/cm 2 .